To speed up the picking of an order, a barcode is printed on the invoice or delivery note. The barcode is displayed below the upper right-hand block, which usually contains the shop's address as well as the customer number, date, etc. The barcode is printed on the invoice or delivery note.
If a red X is displayed instead of the barcode, please check the option allow_url_fopen in the PHP configuration of your server. This value must be true for the barcode to be displayed correctly.
If the barcode is not visible at all, please check whether the HTML element with the ID head_right, which is used to add the barcode, is contained in the invoice template index.tpl you are using. If not, insert such an element into your invoice template.